Digital television services have been offered to subscribers by cable television, satellite and more recently via fiber to the home. In the case of cable networks, the signals received by subscribers are typically RF carriers within the range of about 50 to 900 MHz and each carrier is typically modulated with digital data representing one or more programs. In accordance with ITU-T Recommendation J.83, a quadrature amplitude modulated (QAM) format is commonly used such that each RF carrier transmits data at around 38 megabits per second. The modulating signals carry audio-visual data or other information in the form of MPEG data packets. A single modulated carrier may simultaneously transport packets for several program data streams. MPEG data packets belonging to each given audio or video stream are distinguished from each other by a program ID or ‘PID’ value in each packet. In the industry, it is commonplace to use the term ‘QAM’ as a noun to refer to a quadrature amplitude modulated carrier itself or to the quantity of bandwidth thereof.
One practice now becoming prevalent to make efficient use of the available distribution infrastructure and carrier resources is called “switched digital video” or “switched broadcast”. Conventional broadcasting presents the same programming on a particular dedicated set of channels at all times, which well suits the most popular or mainstream programming such as national network news and primetime shows (from CNN, ABC, CBS, NBC). At any given time, there is likely to be a substantial proportion of viewers watching these programs.
In contrast, switched digital video (SDV) is applied to programming that is less universally popular and may have very specialized, temporary or localized viewership. A switched digital video ‘domain’ or ‘service group’ typically encompasses a small subset of a cable network, reaching only a few hundred or a few thousand households. With this technology, an available program is assigned to a broadcast carrier only when at least one subscriber in a service group ‘tunes’ to the channel, in effect requesting that program. Because this approach uses RF carrier resources only for shows that are actually being watched (in theory), the network operator can rely on the statistical nature of shifting demands to significantly ‘oversubscribe’ the distribution network. The operator can offer a greater variety of programs without using dedicated channels and by counting on a low likelihood that the number of different shows requested at any time will exceed the overall distribution capacity. The operator can also decide how granular the SDV deployment can be, trading off the costs of maintaining more SDV equipment versus agility to better serve a smaller number of localized subscribers per service group.
Typically, channels to be switched are converted to constant bit rate to facilitate freely switching programming onto RF channels. A given “standard-definition” (roughly 720×480 pixels) video program may vary in its encoded bit rate due to fluctuations in scene complexity. Through re-encoding and other techniques well known in the art, a standard-definition digital video signal may be “rate clamped” to around 3.75 Mbps while still preserving good quality. At this rate, ten such programs may be transmitted over single 38.75 Mbps QAM. The remaining bandwidth may go toward framing/coding overhead and ancillary data streams. Similarly, each high-definition program can be clamped to a bit rate of around 16 Mbps, allowing a single QAM carrier to carry two high-definition programs.
At any time, the demand for infrequently used or “long tail” content requested by a given population of subscribers may vary. A given program may be assigned to a QAM/PID (that is, an RF carrier and a subset of packets among the data modulating that carrier) if at least one subscriber is tuned to the channel. The same program transmission may be discontinued when no subscribers remain tuned to the given program. This releases RF bandwidth resources to carry other programs for which there is current subscriber demand. At an edge switching device that directs certain streams to selected QAMs, a given QAM has traditionally been statically configured to either carry only standard-definition programs (up to ten) or to carry high-definition programs (up to two).
As subscribers select different ongoing programs to view or select video on-demand programs, the relative demand between standard-definition programs and high-definition programs may shift. An array of QAMs, some configured to carry standard-definition and others to carry high-definition, may be overbooked with standard-definition streams while other QAMs designated for high-definition may remain partially or fully unused (or vice versa). This leads to inefficient use of QAM bandwidth resources and occasional inability to service some subscribers' requests, despite the availability of QAM resources that happen to be configured for a different per-program bit rate. Consequently, there is a need to allow for QAM resources to be dynamically adapted to different per-program bit rates. In particular, there arises a need to coordinate the rearrangement of QAM resources, including mid-program reassignment of programs to RF channels/PIDs. A subscriber's receiver (e.g., a set-top box, television or similar terminal device) will initially be directed to tune to one RF frequency and PID. Then, if the program is reassigned after the initial tuning, the receiver will need to be notified of the RF channel/PID change and retune accordingly.